1. Field of the Invention
The present invention relates to a semiconductor light source apparatus using a semiconductor light emitting device, for example, a light emitting diode (LED) or laser diode (LD), and a semiconductor light source control method.
2. Description of the Related Art
As an LED drive circuit to reduce power consumption and also to control LED brightness at a constant level, technology including Jpn. Pat. Appln, KOKAI Publication No. 2005-011895 can be considered.
Including the technology described in the above-described document, a drive of a semiconductor light emitting device is generally configured by a linear constant current circuit that gives current feedback by combining an operational amplifier and a power transistor or a constant current control type DC/DC converter.
It is necessary for fast PWM driving to advance a current rise in PWM waveforms. In the former linear constant current circuit, a current overshoot is caused by an overdrive due to a feedback delay. Thus, it is difficult to combine a faster operation with stable feedback control. When a faster operation is realized, the load on a transistor in an output stage is heavy so that it becomes necessary to allow more voltage margins and also to take measures against heating of the transistor.
In the latter DC/DC converter, a time of at least several to a dozen or so switching cycles is needed before the current of an inductor used in the circuit is returned to an original current value. Thus, due to such rise characteristics of the current, it is difficult to achieve a faster operation.
Recently, to improve difficulties described above in the latter DC/DC converter, a DC/DC converter that can achieve a faster operation by temporarily holding a feedback voltage when the PWM waveform is on or an output voltage of a capacitor in the circuit to make the current rise steep when the PWM waveform is on next time. Despite the above efforts, faster PWM cycles limited to about several ten [kHz] and it is difficult to achieve a still faster operation.